1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an In-Plane Switching (IPS) mode liquid crystal display (LCD) device and a method of manufacturing the same.
2. Discussion of the Related Art
Demands for various display devices have increased as the information society has developed. Accordingly, many efforts have been made to research and develop various types of flat display devices, such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Some types of flat display devices have already been used as displays in a variety of different applications. Among the various flat display devices, liquid crystal display (LCD) devices have been most widely used due to the advantageous characteristics of thin profile, light weight, and low power consumption. LCD devices have been provided as a substitute for a Cathode Ray Tube (CRT) in many applications. In addition, mobile type LCD devices, such as a display for a notebook computer, have been developed. Further, LCD devices can be used as computer monitors, televisions or other types of equipment that display video.
Various technical developments an research in LCD technology has been ongoing. However, the picture quality is still, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various fields as a general display, the key to developing LCD devices depends on whether LCD devices can implement a high quality picture, such as a high resolution and high luminance large-sized screen, while still maintaining light weight, thin profile, and low power consumption.
In general, an LCD device includes an LCD panel for displaying a picture image, and a driving part for applying a driving signal to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other with a predetermined gap therebetween. A liquid crystal layer is injected into the gap between the first and second glass substrates.
The first glass substrate (TFT array substrate) includes a plurality of gate and data lines, a plurality of pixel electrodes, and a plurality of thin film transistors. The plurality of gate lines are formed on the first glass substrate at fixed intervals, and the plurality of data lines are formed perpendicular to the plurality of gate lines at fixed intervals. The plurality of pixel electrodes, arranged in a matrix-type configuration, are respectively formed in pixel regions defined by the plurality of gate and data lines that cross each other. The plurality of thin film transistors are switched according to signals from the gate lines to transmit signals of the data lines to the respective pixel electrodes.
The second glass substrate (color filter substrate) includes a black matrix layer that excludes light from regions except for the pixel regions of the first substrate. The second glass substrate also includes an R/G/B color filter layer for displaying various colors. Further, a common electrode can be positioned on the second glass substrate. However, in the case of an In-Plane switching mode LCD device, the common electrode is formed on the first glass substrate.
The gap between the first and second glass substrates is maintained by spacers when the first and second substrates are bonded to each other by a seal pattern having a liquid crystal injection inlet. The liquid crystal layer is formed using a liquid crystal injection method, in which the liquid crystal injection inlet is dipped into a container having liquid crystal while a vacuum state is maintained in the gap between the first and second glass substrates. That is, the liquid crystal is injected between the first and second substrates by an osmotic action. Subsequently, the liquid crystal injection inlet is sealed with a sealant.
A LCD device is driven according to the optical anisotropy and polarizability of liquid crystal. Liquid crystal molecules can impart directional characteristics on light because liquid crystal molecules have long and thin shapes. The directional characteristics of the liquid crystal molecules can be controlled by inducing electric field across the liquid crystal in the direction of an alignment direction for the liquid crystal molecules. That is, if the alignment direction of the liquid crystal molecules is controlled by the induced electric field, the direction of polarized light can be changed by the optical anisotropy of the liquid crystal to thereby display a picture image.
Liquid crystal is classified into positive (+) type liquid crystal having positive dielectric anisotropy and negative (−) type liquid crystal having negative dielectric anisotropy according to electrical characteristics of the liquid crystal. In the positive (+) type liquid crystal, a longitudinal (major) axis of a positive (+) liquid crystal molecule is in parallel to the electric field applied to the liquid crystal. Meanwhile, in the negative (−) type liquid crystal, a longitudinal (major) axis of a negative (−) liquid crystal molecule is perpendicular to the electric field applied to the liquid crystal.
FIG. 1 is an exploded perspective view illustrating a Twisted Nematic (TN) mode LCD device. As shown in FIG. 1, the TN mode LCD device includes a lower substrate 1 and an upper substrate 2 bonded to each other with a gap therebetween, and a liquid crystal layer 3 in the gap between the lower and upper substrates 1 and 2.
The lower substrate 1 includes a plurality of gate lines 4, a plurality of data lines 5, a plurality of pixel electrodes 6, and a plurality of thin film transistors T. The plurality of gate lines 4 are formed on the lower substrate 1 in one direction at fixed intervals and the plurality of data lines 5 are formed perpendicular to the plurality of gate lines 4 at fixed intervals to thereby define a plurality of pixel regions P. A plurality of pixel electrodes 6 are respectively formed in the pixel regions P defined by the plurality of gate and data lines 4 and 5 that cross each other. A plurality of thin film transistors T are respectively formed at crossings of the gate and data lines 4 and 5. Next, the upper substrate 2 includes a black matrix layer 7 that excludes light from regions except for the pixel regions P, R/G/B color filter layers 8 for displaying various colors, and a common electrode 9.
Each of the thin film transistors T include a gate electrode, a gate insulating layer (not shown), an active layer, a source electrode, and a drain electrode. The gate electrode projects from the gate line 4. The gate insulating layer (not shown) is formed over an entire surface of the lower substrate. The active layer is formed on the gate insulating layer above the gate electrode. The source electrode projects from the data line 5, and the drain electrode is formed opposite to the source electrode. The aforementioned pixel electrode 6 is formed of transparent conductive metal having great transmittance, such as indium-tin-oxide (ITO).
In the aforementioned LCD device, liquid crystal molecules of the liquid crystal layer 3 on the pixel electrode 6 are aligned as a result of a signal applied through the thin film transistor T. Light transmittance is controlled according to alignment of liquid crystal to thereby display a picture image. The liquid crystal molecules by driven by an electric field perpendicular to the lower and upper substrates using the common electrode 9 of the upper substrate 2. This method obtains great transmittance and high aperture ratio. Also, it is possible to prevent liquid crystal cells from being damaged by static electricity since the common electrode 9 of the upper substrate 2 serves as the ground. However, in the case of driving the liquid crystal molecules with an electric field that is perpendicular to the lower and upper substrates, it is difficult to obtain a wide viewing angle.
In order to overcome the narrow viewing angle problem of a Twisted Nematic (TN) mode LCD device, an In-Plane switching (IPS) mode LCD device has been proposed. Hereinafter, a related art IPS mode LCD device will be described with reference FIG. 2, FIG. 3A, FIG. 3B, FIG. 4A, FIG. 4B, FIG. 5 and FIG. 6. FIG. 2 is a cross-sectional view schematically illustrating the related art In-Plane switching mode LCD device. In the related art IPS mode LCD device, as shown in FIG. 2, a common electrode 13 and a pixel electrode 12 are formed on the same plane of a lower substrate 10. Then, the lower substrate 10 is bonded to an upper substrate 20 with a gap therebetween. A liquid crystal 3 is positioned between the lower and upper substrates 10 and 20. The liquid crystal 3 is driven by an electric field between the common electrode 13 and the pixel electrode 12 on the lower substrate 10.
FIG. 3A and FIG. 3B respectively illustrate the alignment direction of liquid crystal when a voltage is turned off and turned on in the related art In-Plane switching mode LCD device.
FIG. 3A illustrates the related art IPS mode LCD device when the voltage is turned off in that no electric field is applied in parallel to the lower and upper substrates between the common electrode 13 or the pixel electrode 12. Accordingly, there is no change in alignment of the liquid crystal 3. For example, liquid crystal molecules are basically twisted at 45° to a horizontal direction of the pixel electrode 12 and the common electrode 13.
FIG. 3B illustrates the related art IPS mode LCD device when the voltage is turned on in that an electric field is applied in parallel to the lower and upper substrates between the common electrode 13 and the pixel electrode 12. Accordingly, alignment of the liquid crystal 3 is changed. In more detail, the alignment of liquid crystal 3 is twisted more at 45° as compared to the alignment of liquid crystal when the voltage is turned off. In this state, the horizontal direction of the common and pixel electrodes 13 and 12 is identical to the twisted direction of liquid crystal.
As mentioned above, the related art IPS mode LCD device has the common electrode 13 and the pixel electrode 12 on the same plane. The related art IPS mode LCD device has the advantageous characteristic of as wide viewing angle. For example, along a front direction of the IPS mode LCD device, a viewer can have a viewing angle of 70° in all directions (i.e., lower, upper, left, and right directions). Furthermore, the related art IPS mode LCD device has simplified fabrication process steps, and reduced color shift. However, the related art IPS mode LCD device has the problems of low light transmittance and low aperture ratio since the common electrode 13 and the pixel electrode 12 are formed on the same substrate. Further, the related art IPS mode LCD device also has the problems of high driving voltages to improve response times, and the need to maintain a uniform cell gap due to the small misalignment margin of the cell gap. That is, the IPS mode LCD device has the aforementioned advantages and disadvantages as compared to the Twisted Nematic (TN) mode LCD device, whereby a user can select the mode of the LCD device according to a purpose.
FIG. 4A and FIG. 4B are perspective views illustrating operation of the IPS mode LCD device while respectively in the states of being turned off and turned on. FIG. 4A is a state when a voltage is not supplied to the pixel electrode 12 or the common electrode 13 such that the alignment direction 16 of the liquid crystal molecules is identical to the alignment direction of an initial alignment layer (not shown). Then, as shown in FIG. 4B, when the voltage is supplied to the pixel electrode 12 and the common electrode 13, the alignment direction 16 of the liquid crystal molecules is corresponding to an electric field application direction 17.
FIG. 5 is a plane view illustrating a unit pixel of the related art IPS mode LCD device. FIG. 6 is a cross-sectional view taken along lines I-I′ and II-II′ of FIG. 5. As shown in FIG. 5 and FIG. 6, the related art IPS mode LCD device includes a transparent lower substrate 60 having a plurality of gate lines 61 and data lines 64 crossing each other to define pixel regions, and a plurality of thin film transistors T respectively where the plurality of gate lines 61 and data lines 64 cross each other. Each of the thin film transistors T includes a gate electrode 61a protruding from the gate line 61, a gate insulating layer 62 over an entire surface of the lower substrate 60 including the gate electrode 61a, an active layer on the gate insulating layer 62 above the gate electrode 61a, a source electrode 64a protruding from the data line 64, and a drain electrode positioned at a predetermined interval from the source electrode 64a. Also, a common line 61b is formed in the same layer as the gate line 61. More particularly, the common line 61b is formed in parallel to the gate line 61 within the pixel region.
A passivation layer 65 is formed over the entire surface of the lower substrate 60 including the data line 64, and a contact hole 66 is formed to expose the drain electrode 64b. The passivation layer 65 is formed of silicon nitride. Then, a common electrode 67 and a pixel electrode 68 are alternately formed on the passivation layer 65 of the pixel region in parallel. The common electrode 67 is connected to the common line 61b through the contact hole 69, and the plurality of common electrodes 67 are formed in parallel to the data line 64 within one pixel region. The pixel electrode 68 is connected to the drain electrode 64b of the thin film transistor through the contact hole 66. Both the common electrode 67 and the pixel electrode 68 are formed of transparent conductive layers.
Although not shown, an upper substrate is formed opposite to the lower substrate. The upper substrate includes color filter layers corresponding to the pixel regions of the lower substrate, and a black matrix layer for preventing light leakage on the portions except the pixel regions. At this time, the black matrix layer is formed corresponding to the portions including the gate line 61, the data line 64, the common electrode 67 adjacent to the data line 64, and the thin film transistor. Also, liquid crystal molecules positioned between the common electrode 67 and the pixel electrode 68 are aligned in the same direction as that of an electric field parallel to the substrates between the common electrode 67 and the pixel electrode 68, thereby forming one domain.
As mentioned above, the common electrode 67 and the pixel electrode 68 are formed of transparent conductive layers. Luminance is improved by using the transparent conductive layers. However, when the black matrix layer is formed on the data line 64 and the adjacent portions, it is necessary to take a margin for bonding the lower and upper substrate into consideration, thereby complicating manufacturing process steps. Also, depending on the resolution, a bonding margin can cause a decrease in luminance at the periphery of the data line 64. In other words, the black matrix layer is formed on the corresponding portion between the common electrodes 67 adjacent to the data line 64 as well as the data line 64, thereby causing a decrease of the aperture ratio and the luminance by the bonding margin.
In addition, the passivation layer is formed of silicon nitride having a thickness of approximately 0.3 μm. Such a silicon nitride thickness may result in cross-talk between the data line and the common electrode, and a deterioration of the picture quality by parasitic capacitance may also occur. Accordingly, in order to prevent cross-talk problem and problems of parasitic capacitance, an organic insulating layer having a low dielectric constant is formed over the entire surface of the lower substrate instead of a silicon nitride passivation layer. However, such an an organic insulating layer may cause the problem of decreased light transmission efficiency due to the thick organic insulating layer. That is, while the silicon nitride layer is formed having a thickness of approximately 0.3 μm, the organic insulating layer is formed having a thickness of approximately 3 μm, so that the light transmission efficiency of the pixel region is lowered to approximately 92% by approximately a 8% light transmission efficiency decrease.